Laboratory tests are a vital part of the accurate diagnosis of a patientâs condition, with around 1 billion tests performed in the UK each year. To enable the routine application of predictive, preventive and personalized healthcare, these biochemical tests will have to be performed at a much larger scale, at much lower cost, and preferably at point-of-care locations rather than at clinical laboratories. The realization of this highly desirable situation necessitates the development of new, more cost-effective, technologies for biosensor fabrication. Recently, silicon nanowire biosensors have been reported suitable for real-time, high sensitivity, high selectivity and label-free biosensing. The high sensitivity is due to the nanoscale diameter of nanowires, which is comparable to that of biomolecules. However, while the feasibility of nanowire biosensors has been demonstrated, current fabrication approaches are expensive due to the use of silicon-on-insulator wafers and e-beam lithography.
This project aims to develop a low-cost fabrication process for silicon nanowire biosensors using thin film transistor technology. The use of low cost glass or plastic substrates requires a low thermal budget process for the nanowire biosensor fabrication. Amorphous silicon could be used to fabricate the silicon nanowires, but the mobility is very low. In this project, nickel-induced lateral crystallization is being researched to convert amorphous silicon into polycrystalline silicon using a low temperature anneal. Polycrystalline silicon has a dramatically higher value of mobility than amorphous silicon and hence should give better sensor performance. Two nanowire configurations are being investigated, namely Si-on-Oxide and Si-on-Air. The Si-on-Oxide structure has the advantage of a simple fabrication process, whereas the Si-on-Air structure has the advantage of allowing biomolecule attachment all around the nanowire and hence should have higher sensitivity. Fig.1(a) shows that crystallization proceeds more rapidly in the Si-on-Air structure, which should allow crystallization to be achieved at a lower temperature. We are also investigating the use of fluorine implantation to enhance the crystallization. Fig.1 (b) shows that fluorine aids crystallization at or below a critical dose of 1E15 cm-2.